1. Field of the Invention
This invention relates to an angular velocity detecting apparatus, and particularly to an apparatus for detecting rotational angular velocity by the utilization of a ring laser type gyro.
2. Related Background Art
A mechanical gyro having a rotor or a vibrator and an optical gyro are known as gyros for detecting the angular velocity of a moving object. Particularly the optical gyro is capable of starting in a moment and has a wide dynamic range and therefore is bringing about an innovation in the technical field of gyro. Optical gyros include a ring laser type gyro, an optical fiber gyro, a passive type ring resonator gyro, etc. Among these, the ring laser type gyro using a gas laser is the one of which the development has been started earliest, and has already been put into practical use in aircraft, etc. Recently, as a compact and highly accurate ring laser type gyro, there is a semiconductor laser gyro integrated on a semiconductor substrate.
FIG. 12 of the accompanying drawings is a plan view of an example of the optical gyro which can detect not only the magnitude of angular velocity but also a direction of rotation. The reference numeral 10 designates a quartz tube, the reference numeral 11 denotes the asymmetrical tapered portion of a light waveguide, the reference numeral 12 designates a mirror, the reference numeral 13 denotes an anode, the reference numeral 14 designates an electrical terminal, the reference numeral 15 denotes a cathode, the reference numeral 100 designates a counter-clockwise laser beam, and the reference numeral 110 denotes a clockwise laser beam.
It is to be understood here that the wavelength of a first laser beam travelling round clockwisely is xcex1. Also, it is to be understood that the wavelength of a second laser beam travelling round counter-clockwisely is xcex2 ( less than xcex1) When the laser is rotated clockwisely, the oscillation frequency f1 of the clockwise first laser beam decreases by                               Δ          ⁢                      xe2x80x83                    ⁢                      f            1                          =                                            2              ⁢                              S                1                                                                    λ                1                            ⁢                              L                1                                              ⁢          Ω                                    (        1        )            
as compared with the oscillation frequency f10, during the non-rotation. Here, S1 is the closed area surrounded by the optical path of the first laser beam, L1 is the length of the optical path of the first laser beam, and xcexa9 is the angular velocity of the rotation. On the other hand, the oscillation frequency f2 of the counter-clockwise second laser beam increased by                               Δ          ⁢                      xe2x80x83                    ⁢                      f            2                          =                                            2              ⁢                              S                2                                                                    λ                2                            ⁢                              L                2                                              ⁢          Ω                                    (        2        )            
as compared with the oscillation frequency f20 during the non-rotation. Here, S2 is the closed area surrounded by the optical path of the second laser beam, and L2 is the length of the optical path of the second laser beam. At this time, the first laser beam and the second laser beam coexist in the laser. Accordingly, a beat light having the difference between the oscillation frequencies of the first laser beam and the second laser beam, etc.,                                           f            2                    -                      f            1                          =                                            f              20                        -                          f              10                        +                          (                                                Δ                  ⁢                                      xe2x80x83                                    ⁢                                      f                    2                                                  +                                  Δ                  ⁢                                      xe2x80x83                                    ⁢                                      f                    1                                                              )                                =                                    f              20                        -                          f              10                        +                          (                                                                                          2                      ⁢                                              S                        2                                                                                                            λ                        2                                            ⁢                                              L                        2                                                                              ⁢                  Ω                                +                                                                            2                      ⁢                                              S                        1                                                                                                            λ                        1                                            ⁢                                              L                        1                                                                              ⁢                  Ω                                            )                                                          (        3        )            
is created in the laser. On the other hand, during the counter-clockwise rotation, a beat light having a frequency indicated in the following expression (4) is created.                                           f            2                    -                      f            1                          =                                            f              20                        -                          f              10                        -                          (                                                Δ                  ⁢                                      xe2x80x83                                    ⁢                                      f                    2                                                  +                                  Δ                  ⁢                                      xe2x80x83                                    ⁢                                      f                    1                                                              )                                =                                    f              20                        -                          f              10                        -                          (                                                                                          2                      ⁢                                              S                        2                                                                                                            λ                        2                                            ⁢                                              L                        2                                                                              ⁢                  Ω                                +                                                                            2                      ⁢                                              S                        1                                                                                                            λ                        1                                            ⁢                                              L                        1                                                                              ⁢                  Ω                                            )                                                          (        4        )            
When two or more oscillation modes exist in the laser, the inverted population exhibits a time fluctuation conforming to the difference between the oscillation frequencies of the modes. This phenomenon is known as the pulsation of inverted population. In the case of a laser letting an electric current flow such as a gas laser or a semiconductor laser, there is the correspondence relation of 1 to 1 between the inverted population and the impedance of the laser. When lights interfere with each other in the laser, the inverted population changes in conformity therewith and as the result, the impedance between the electrodes of the laser changes. The manner of this change appears as a change in a terminal current if a constant voltage source is used as a driving power source. Also, if a constant current source is used, the manner of interference between lights can be taken in the form of a signal as a change in a terminal voltage. Of course, any change in the impedance can also be directly measured by an impedance meter.
Accordingly, by providing a terminal for detecting any change in the electric current, voltage or impedance of the laser, a beat signal conforming to rotation can be taken out from this terminal. Further, as shown in expressions (3) and (4), the beat frequency increases or decreases in conformity with the direction of rotation.
Accordingly, by observing an increase or decrease in the beat frequency from during the non-rotation, the direction of rotation can be detected. It is when the difference between the oscillation frequencies satisfies the following expression (5) that the direction of rotation can be detected.
f2xe2x88x92f1xe2x89xa70xe2x80x83xe2x80x83(5)
If the oscillation wavelengths of the first laser beam and the second laser beam are equal to each other,
f20xe2x88x92f10=0xe2x80x83xe2x80x83(6)
and the beat frequency f2xe2x88x92f1 assumes positive and negative signs. If the absolute values of the beat frequencies are equal to each other, the same signal is outputted from terminals and therefore, in this case, the direction of rotation cannot be detected.
In contrast, if design is made such that the signs of the beat frequencies are always the same (in the description, the sign is taken as positive) and the absolute values thereof change depending on the direction of rotation, the detection of the direction of rotation will become possible.
Now, to change the oscillation threshold values of the laser beams propagating round in opposite directions of rotation in the ring laser, loss can be given to only the light propagating round in one direction of rotation. For example, by providing a tapered portion of an asymmetrical shape on a portion of the light waveguide, the total reflection condition deviates relative to a light incident on this tapered portion. Therefore, mirror loss occurs to the light incident on the tapered portion. The angle of incidence onto the tapered portion differs depending on the direction of rotation of the light and therefore, loss can be made great to the laser beam propagating round in a certain direction and loss can be made small to the light propagating round in the opposite direction. As the result, the oscillation threshold value can be changed for the laser beams propagating round in opposite directions of rotation.
Now, it is known that when two modes coexist, there are relations shown in the following expressions (7) and (8) between the oscillation frequency fi and photon number density Si (i=1, 2).
2xcfx80f1+"PHgr"1=xcexa91+"sgr"1xe2x88x92xcfx811S1xe2x88x92xcfx8412S2xe2x80x83xe2x80x83(7)
2xcfx80f2+"PHgr"2=xcexa92+"sgr"2xe2x88x92xcfx812S2xe2x88x92xcfx8421S1xe2x80x83xe2x80x83(8)
Here, "PHgr"i is a phase, xcexa9i is an resonance angle frequency, "sgr"i is a coefficient representative of the entrainment of the mode, xcfx81i is a coefficient representative of the self-extrusion of the mode, and xcfx84ij is a coefficient indicative of the mutual extrusion of the modes. Here, i, j=1, 2; ixe2x89xa0j. Since the oscillation threshold value differs, the photon number density Si (i=1, 2) differs. Accordingly, in accordance with expressions (7) and (8), a difference can be given between the oscillation frequencies.
Specifically, for example, in the above-described construction, a quartz block is hollowed out by the use of a drill to thereby form the quartz tube 10. Thereafter, the mirror 12 is attached to the quartz tube 10. The anode 13, the electrical terminal 14 and the cathode 15 are further attached to the quartz tube 10. Next, helium gas and neon gas are introduced into the quartz tube 10, and when a voltage is applied between the anode and the cathode, discharge begins and an electric current comes to flow. Then, the counter-clockwise laser beam 100 and the clockwise laser beam 110 oscillate in the quartz tube 10.
When the quartz tube 10 is stationary, the oscillation frequencies of the laser beam 100 and the laser beam 110 are substantially equal to each other, i.e., 4.73xc3x971015 Hz, and the oscillation wavelength xcex thereof is 632.8 nm. However, the tapered portion 11 of the optical waveguide is of an asymmetrical shape and therefore, the oscillation threshold value for the laser beam 100 is smaller than the oscillation threshold value for the laser beam 110. Therefore, the light intensity of the laser beam 100 is greater than the light intensity of the laser beam 110. As the result, the oscillation frequency f1 of the laser beam 100 is greater by 20 MHz than the oscillation frequency f2 of the laser beam 110. The laser beams 100 and 110 interfere with each other in the quartz tube 10. At this time, the power source current is adjusted so as to become constant, and when the voltage between the electrode terminal 14 and the cathode 15 is monitored, a signal of an amplitude 100 mV and a frequency 20 MHz is obtained. That is, even when the quartz tube 10 is stationary, a beat voltage can be detected.
Now, when the quartz tube 10 is clockwisely rotated at a speed of 1 degree per second and the length of a side of a resonator is 10 cm, the oscillation frequency f1 of the counter-clockwise laser beam 100 increases by 248.3 kHz. On the other hand, the oscillation frequency f2 of the clockwise laser beam 110 decreases by 248.3 kHz. Accordingly, the beat frequency becomes (f1xe2x88x92f2)=20 MHz+496.6 kHz. On the other hand, when the quartz tube 10 is counter-clockwisely rotated at a speed of 1 degree per second, the beat frequency becomes (f1xe2x88x92f2)=20 MHzxe2x88x92496.6 kHz. The absolute value of the amount of increase or decrease of the beat frequency is proportional to the rotating speed and therefore, not only the detection of the rotating speed is possible, but also the detection of the direction of rotation becomes possible because the direction of rotation and the increase or decrease in the beat frequency correspond to each other at 1 to 1.
In this case, constant current driving has been used and a change in the terminal voltage has been measured, but if constant voltage driving is used, any change in the electric current flowing through the terminal can be detected. Also, any change in the impedance of discharge may be directly detected by the use of an impedance meter.
Thereby, a photodetector for detecting the beat light becomes unnecessary and as the result, return light noise which may be caused by the return light from the photodetector becomes null.
While helium gas and neon gas have been used herein, any gas which may be laser-oscillated may be used. Also, the shape surrounded by the optical path of the light waveguide is not limited to a square, but may be a shape such as a hexagon, a triangle or a circle.
FIG. 13A of the accompanying drawings shows an example of the change in angular velocity to which the gyro is subjected, and the side on which the angular velocity xcexa9 is positive relative to 0 is the clockwise direction, and the side on which the angular velocity xcexa9 is negative relative to 0 is the counter-clockwise direction. In the example of the angular velocity indicated by solid line 51, it is represented that with the lapse of time, the angular velocity in the clockwise direction changes to the angular velocity in the counter-clockwise direction.
FIG. 13B of the accompanying drawings shows an example of the change in the terminal voltage Vg of the gyro for a change in angular velocity indicated by 51, and it is represented that as indicated by curve 52, the frequency of a change in beat voltage appearing in the terminal voltage by the interference between the laser beams changes to a low frequency side with a change in an angular velocity.
FIG. 13C of the accompanying drawings shows the voltage waveform of a rectangular wave obtained by comparing the terminal voltage indicated by this curve 52 at the level of Vref of FIG. 13C. Information regarding the angular velocity can be obtained if the time interval of e.g. the rising edge of the voltage 53 of this rectangular wave or the number of edges measured within a predetermined time is measured.
FIG. 14 of the accompanying drawings shows the relation between the angular velocity applied to the gyro and the frequency of the beat voltage, and the beat frequency at the angular velocity 0 is defined as f0. In FIG. 14, the maximum angular velocity in the necessary range for detection is represented as xcexa9mx, and the beat frequency in that case is represented as fmx, and the minimum angular velocity in the necessary range for detection is represented as xcexa9mn, and the beat frequency in that case is represented as fmn. xcexa90 is the angular velocity at which the beat frequency becomes 0. It is indicated by a straight line 54 that when the angular velocity increases in the clockwise direction, the beat frequency becomes high, and when the angular velocity increases in the counter-clockwise direction, the beat frequency becomes low. In apparatuses incorporating and utilizing the gyro (for example, a camera, binoculars, a navigation apparatus, etc.), there are the necessary ranges for detection and necessary detection resolution of angular velocity required in conformity with the respective apparatuses. As an example, when angular velocity detection for preventing hand vibration is to be effected with a gyro mounted on a still camera, it is known that it is enough if angular velocity in the range of about xe2x88x9220 to +20 degrees/sec. can be detected at resolution of the order of 0.1 degree/sec. In the above-described example of the camera, xcexa9mx=20 degrees/sec. and xcexa9mn=xe2x88x9220 degrees/sec. Also, there are frequency characteristics, etc. required in conformity with the characteristics of the apparatuses incorporating and utilizing the gyro, and as an example of the case of a system for preventing the hand vibration of the still camera, it is necessary that as the overall frequency characteristic of the system, the order of DCxe2x80x94100 Hz be a band and particularly, in the range of DCxe2x80x9430 Hz, the phase delay be xe2x88x9215xc2x0 or less. It is also known that when a system for the prevention of the hand vibration of a still camera satisfying such a frequency characteristic is to be built by digital control, the detection of the angular velocity by the hand vibration must be effected at each interval of at least about 1 msec. with the time required for digital calculating process, etc. taken into account.
Here, in a ring laser gyro, the information of angular velocity is represented by a change in the frequency of the terminal voltage and therefore, unless the change in the frequency of the terminal voltage is detected at a time interval whereat the angular velocity is detected, the frequency characteristic of the above-described system cannot be satisfied. Specifically, in a system wherein the detection of angular velocity need be effected at each interval of 1 msec., it is necessary that in the whole of the necessary range for the detection of the angular velocity, it is necessary to make the beat frequency the ring laser gyro outputs to the terminal voltage higher than at least 1 kHz (shorter than 1 msec. as the cycle).
Also, even if there is adopted a processing circuit for converting the change in frequency by a change in angular velocity into a change in voltage by an FV conversion circuit and effecting the detection of the angular velocity, an analog circuit such as the FV conversion circuit includes such parts difficult to integrate as a capacitor and a resistor, and particularly when such an angular velocity detecting apparatus is to be mounted on a compact electronic apparatus such as a camera, it is not always suitable in terms of the space or cost of the processing circuit.
So, this invention has as its task to provide an angular velocity detecting apparatus suitable for being made into an integrated circuit and capable of accurately accomplishing angular velocity detection.
One aspect of this invention is an angular velocity detecting apparatus provided with a ring laser including a tapered light waveguide having an asymmetrical shape in at least a portion of the light waveguide so that the oscillation threshold values of laser beams propagating round in opposite directions of rotations may differ from each other, an optical gyro having a terminal for detecting any change in the electric current, voltage or impedance of the ring laser, a measuring device for measuring the information of the cycle of the change in the electric current, voltage or impedance outputted from the terminal of the optical gyro, a clock generating device for generating a predetermined sampling clock, and a calculation circuit for inputting the result of the measurement by the measuring device at a predetermined sampling cycle, and calculating information regarding angular velocity on the basis of the result of the measurement, and within the necessary range for the detection of the angular velocity, the cycle of the change in the electric current, voltage or impedance produced by the optical gyro is made shorter than the sampling cycle.
One aspect of this invention is an angular velocity detecting apparatus provided with the above-described ring laser, the above-described optical gyro, a measuring device for measuring the information of the frequency of any change in an electric current, a voltage or impedance outputted from the terminal of the optical gyro, a clock generation device for generating a predetermined clock for sampling, and a calculation circuit for inputting the result of the measurement by the measuring device at a predetermined sampling frequency, and calculating information regarding angular velocity on the basis of the result of the measurement, and within the necessary range for the detection of the angular velocity, the frequency of the change in the electric current, voltage or impedance produced by the optical gyro is made higher than the sampling frequency.